In many feeding and finishing modules in marking systems including electrostatic marking systems, a stack of sheets is generally being processed. The topography of the top of the stack is often a critical but uncontrolled factor. If the top of the stack is excessively non-planar in a feeder module, then feeder acquisition failures may occur. Stack non-planarity in a finisher may be indicative of poor quality of finished documents and may result in customer dissatisfaction in the end article. The predominant cause of stack surface non-planarity is sheet curl. Products within the high-end office and production markets typically have some decurling capability. In general, the system operator needs to confirm that the decurling capability is set appropriately for the media type and job type. This may involve visual inspection of the stack quality as it is formed within the finishing module. The operator is thus forced to ‘close the loop’ to assure that output quality will be acceptable to the end customer. A less subjective and labor intensive process than now exists would be preferable.
Various means have been suggested to measure the degree of curl and flatness at the top of the paper stack. This is important since sheet curl causes problems of handling as the sheet is processed in the feeding, printing or finishing module. Sheets delivered in a curled condition have a tendency to have their edges out of registration with the aligning mechanisms and other sheet moving systems employed in the printing machine. In addition, curled sheets tend to frequently produce jams or misfeeds within the feeding and printing modules, and especially within output sorting, stacking, collating, compiling and/or other sheet handling systems. Belts or soft rollers may be used in conventional prior art decurlers in which one or more paper paths are effective in reducing paper curls primarily in the direction of sheet travel . However, the setting of such decurlers is complex since paper curl is a function of many parameters. Furthermore, efficient and accurate measurement of paper curl of individual sheets as they are being processed is difficult, so that information needed to define the setting of decurlers is not readily available. By contrast, a measure of paper curl of a stack being formed within an output device provides an averaging effect over multiple sheets, thus enabling a more reliable, less noisy estimate of the sheet curl level. In a similar fashion, a measure of paper curl of a stack being drawn from within an input device enables the feeding mechanism to make adjustments to preferentially feed curled sheets.
A reliable, efficient method to measure the degree of curl on the top of a paper stack is therefore important to make a sheet processing system less sensitive to the effects of paper curl.